THE GENERAL ENTOMOLOGICAL ECOLOGY OF

THE INDIAN CORN PLANT

PROFESSOR S. A. FORBES

ILLINOIS STATE ENTOMOLOGIST

ECOLOGY being the science of the interactions between an organism, or a group of organisms, and its environment, and between organisms in general and their environment in general, this complex of relations may, of course, be divided in various ways. The division here used implies a centripetal grouping of the facts of relationship around single kinds of organisms, and the group of facts to be discussed is that of which the corn plant is the center and the insects of its environment are the active factors.

A prolonged study, extending over many years, of the entomology of the corn plant, the economic results of which have been published in my seventh and twelfth reports as State Entomologist of Illinois (the Eighteenth and Twenty-third of the office series) has left in my possession a considerable body of information capable of treatment from the standpoint of pure ecology, and the beginnings of such a treatment are here assembled because of the rising interest in ecological investigation and the promise which it gives of interesting and important results, and because of a wish to illustrate, in some measure, the general scientific value of such materials, of which, it scarcely need be said, the economic entomologists of this country have accumulated a large amount.

INSECT INFESTATION OF THE CORN PLANT

We know of some two hundred and twenty-five species of insects in the United States which are evidently attracted to the corn plant because of some benefit or advantage which they are able to derive from it. The

principal groups of this series are ninety species of Coleoptera, fifty-six species of larvæ of Lepidoptera, forty-five species of Hemiptera and twenty-five species of Orthoptera. The other insect orders are represented by seven or eight species of Diptera and one or two of Hymenoptera. Every part of the plant is liable to infestation by these insects, but the leaves and the roots yield the principal supplies of insect food, either in the form of sap and protoplasm sucked from their substance by Hemiptera, or in that of tissues and cells devoured by the subterranean larvæ of Coleoptera, and by caterpillars, grasshoppers and beetles, feeding above ground.

LACK OF SPECIAL ADAPTATIONS

Notwithstanding the great number of these insects, and the variety and importance of the injuries which they frequently inflict upon the corn plant, there is little in its structure or its life history to suggest any special adaptation of the plant to its insect visitants-no lure to insects capable of service to it, or special apparatus of defense against those able to injure it. The fertilization of its seed is fully provided for without reference to the agency of insects. It has no armature of spines or bristly hairs to embarrass their movements over its surface or to defend against their attack its softer and more succulent foliage. It secretes no viscid fluids to entangle them, and forms no chemical poisons or distasteful compounds in its tissues to destroy or to repel them. The cuticle of its leaf is neither hardened nor thickened by special deposits; its anthers are neither protected nor concealed; and its delicate styles are as fully exposed as if they were the least essential of its organs. Minute sucking insects are able at all times to pierce its roots and its leaves with their flexible beaks, and with the single exception of its fruit there is no part of it which is not freely accessible at any time to any hungry enemy. Only the kernel, which is supposed to have been lightly covered in the wild corn plant by a single chaffy scale or glume, has

become, in the long course of development, securely inclosed beneath a thick coat of husks, impenetrable by nearly all insects; and we may perhaps reasonably infer that, among the possible injuries against which this conspicuous protective structure defends the soft young kernel, those of insects are to be taken into account.

There are, of course, many insect species, even among those which habitually frequent the plant, which are unable to appropriate certain parts of its substance to their use, but this is because of the absence of adaptation on their part and not because of any special defensive adaptation on the side of the plant. Thus we may say that, with the exception of the ear, the whole plant lies open and free to insect depredation, and that it is able to maintain itself in the midst of its entomological dependents only by virtue of its unusual power of vigorous, rapid and superabundant growth. Like every other plant which is normally subject to a regular drain upon its substance from insect injury, it must grow a surplus necessary for no other purpose than to appease its enemies; and this, in a favorable season, the corn plant does with an energetic profusion unexampled among our cultivated plants. Insects, indeed, grow rapidly as a rule, and most of them soon reach their full size. Many species multiply with great rapidity, but even these the corn plant will outgrow, if given a fair chance, provided they are limited to corn itself for food.

Turning to the other side of the relationship, we may say that the corn insects exhibit no structural adaptations to their life on the corn plant-no structures, that is to say, which fit them any better to live and feed on corn than on any one of many other kinds of vegetation. This was, of course, to be expected of the great list of insects which find in corn only one element of a various food, and that not necessarily the most important; but it seems equally true of those which, like the corn rootworm or the corn root-aphis, live on it by strong preference, if not by absolute necessity.

Aphis maidiradicis, the so-called corn root-aphis, is not especially different in adaptive characters from the other root-lice generally, and it lives, indeed, in early spring, on plants extremely unlike corn. Finding its first food on smartweed (Polygonum), and on the field grasses (Setaria, Panicum, etc.), it is scarcely more than a piece of good fortune for it and for its attendant ants if the ground in which it hatches is sometimes planted to corn, in which it finds a more sustained and generous foodsupply than in the comparatively small, dry and slowgrowing plants to which it would otherwise be restricted.

The larva of Diabrotica longicornis, usually known as the corn root-worm, is, of course, well constructed to burrow young corn roots, but it differs from related Diabrotica larvæ in no way that I know of to suggest a special adaptation to this operation, except in the mere matter of size. If it were larger it would probably eat the roots entire, as does the closely related and very similar larva of D. 12-punctata. Indeed, there is some reason to believe that D. longicornis may breed in large swamp grasses, since the beetle has been found abundant in New Brunswick in situations where it is difficult to suppose that it originated in fields of corn, and where such grasses are extremely common. Even the special corn insects seem, in short, structurally adapted to much more general conditions than those supplied by the corn plant alone, and if they are restricted largely or wholly to this plant for food, this seems due to other conditions than those supplied by special structural adaptations.

In short, in the entomological ecology of the corn plant we see nothing whatever of that nice fitting of one thing to another, specialization answering to specialization, either on the insect side or on that of the plant, which we sometimes find illustrated in the relations of plants and insects. The system of relations existing in the corn field seems simple, general and primitive, on the whole, like that which doubtless originally obtained between plants in general and insects in general in the early stages of their association.

Such adaptations to corn as we get glimpses of are almost without exception adaptations to considerable groups of food plants, in which corn is included-some of these groups select and definite, like the families of the grasses and the sedges, to which the chinch-bug is strictly limited, and others large and vague, like the almost unlimited food resources of the larvæ of Lachnosterna and Cyclocephala under ground. These are evidently adaptations established without any reference to corn as a food plant, most of them very likely long before it became an inhabitant of our region, and applying to corn simply because of its resemblance, as food for insects, to certain groups of plants already native here. ENTOMOLOGICAL ECOLOGY OF CORN AND THE STRAWBERRY

Corn being, in fact, an exotic or intrusive plant which seems to have brought none-or at most but one1-of its native insects with it into its new environment, it will be profitable to compare the entomological ecology of this introduced but long-established and widely cultivated plant with that of some native species which is also generally and, in some districts, extensively grown.

We may take, for this purpose, the strawberry plant, whose insect visitants and injuries I studied carefully several years ago. About fifty insects species are now listed as injurious to the strawberry, and about twenty of these also infest corn. Two fifths of the known strawberry insects are thus so little specialized to that food that they feed on other plants as widely removed from the strawberry as is Indian corn. On the other hand, six species, all native, are found, so far as known, only, or almost wholly, on the strawberry, at least in that stage in which they are most injurious to that plant. These are the strawberry slug (Emphytus maculatus); the strawberry leaf-roller (Phoxopteris comptana) — occasionally abundant on blackberry and raspberry, to which it spreads from infested strawberry plants adjacent; two 1 Diabrotica longicornis Say.